1. Field of the Invention
The present invention is broadly concerned with anti-reflective compositions and methods of forming the compositions for use as anti-reflective coating (ARC) layers on substrates during integrated circuit manufacturing processes. More particularly, the inventive compositions are formed by polymerizing aminoplasts (e.g., melamine, benzoguanamine) in an acidic environment under elevated temperatures to yield cross-linkable, UV absorbing, fast etching compositions.
2. Description of the Prior Art
A frequent problem encountered by photoresists during the manufacturing of semiconductor devices is that activating radiation is reflected back into the photoresist by the substrate on which it is supported. Such reflectivity tends to cause blurred patterns which degrade the resolution of the photoresist. Degradation of the image in the processed photoresist is particularly problematic when the substrate is non-planar and/or highly reflective. One approach to address this problem is the use of a bottom anti-reflective coating (BARC) applied to the substrate beneath the photoresist layer.
Fill compositions which have high optical density at the typical exposure wavelengths have been used for some time to form these BARC layers. The BARC compositions typically consist of an organic polymer which provides coating properties and a dye for absorbing light. The dye is either blended into the composition or chemically bonded to the polymer. Thermosetting BARC""s contain a cross-linking agent in addition to the polymer and dye. Cross-linking must be initiated, and this is typically accomplished by an acid catalyst present in the composition. As a result of all these ingredients which are required to perform specific and different functions, prior art BARC compositions are fairly complex.
U.S. Pat. No. 5,939,510 to Sato et al. discloses a BARC composition which comprises a UV absorber and a cross-linking agent. The UV absorber is a benzophenone compound or an aromatic azomethine compound having at least one unsubstituted or alkyl-substituted amino group on the aryl groups. The cross-linking agent disclosed by Sato et al. is a melamine compound having at least two methylol groups or alkoxymethyl groups bonded to the nitrogen atoms of the molecule.
The Sato et al. composition suffers from two major drawbacks. First, in the two-component composition disclosed, the Sato et al. composition does not include a polymeric material thus resulting in insufficient coverage on the surfaces and edges of the substrate features. Furthermore, the UV absorber disclosed by Sato et al. is physically mixed with the cross-linking agent rather than chemically bonded to some component of the composition. As a result, the UV absorber will often sublime, and in many cases sublime and diffuse into the subsequently applied photoresist layer.
There is a need for a less complex anti-reflective composition which provides high reflection control and increased etch rates while minimizing or avoiding intermixing with photoresist layers.
The present invention overcomes these problems by broadly providing improved anti-reflective compositions which are formed from a minimal number of components (e.g., two or less) and which exhibit the properties necessary in an effective BARC composition.
In more detail, anti-reflective compositions according to the invention include polymers comprising monomers derived from compounds of Formula I and mixtures thereof. 
wherein each X is individually selected from the group consisting of NR2 (with the nitrogen atom being bonded to the ring structure) and phenyl groups, where each R is individually selected from the group consisting of hydrogen, alkoxyalkyl groups, carboxyl groups, and hydroxymethyl groups. Preferred compounds of Formula I include the following: 
When used in reference to Formula I, the phrase xe2x80x9cmonomers derived from compounds of Formula Ixe2x80x9d is intended to refer to functional moieties of Formula I. For example, each of the structures of Formula II is derived from compounds of Formula I. 
wherein: each X is individually selected from the group consisting of NR2 (with the nitrogen atom being bonded to the ring structure) and phenyl groups, where each R is individually selected from the group consisting of hydrogen, alkoxyalkyl groups, carboxyl groups, and hydroxymethyl groups; and xe2x80x9cM1xe2x80x9d and xe2x80x9cM2xe2x80x9d represent a molecule (e.g., a chromophore or another monomer derived from the compound of Formula I) bonded to Xxe2x80x2 or Xxe2x80x3. Thus, xe2x80x9cmonomers derived from the compounds of Formula Ixe2x80x9d would include those compounds where any of the constituents (i.e., any of the X groups, and preferably 1-2 of the X groups) is bonded to another molecule.
The polymerized monomers are preferably joined by linkage groups selected from the group consisting of xe2x80x94CH2xe2x80x94, xe2x80x94CH2xe2x80x94Oxe2x80x94CH2, and mixtures thereof, with the linkage groups being bonded to nitrogen atoms on the respective monomers. For example, Formula III demonstrates two methoxymethylated melamine moieties joined via a xe2x80x94CH2xe2x80x94 linkage group and two methoxymethylated melamine moieties joined via a xe2x80x94CH2xe2x80x94Oxe2x80x94CH2xe2x80x94 linkage group. 
Formula IV illustrates two benzoguanamine moieties joined via CH2 linkage groups. 
Finally, Formula V illustrates two methoxymethylated melamine moieties having a chromophore (2,4-hexadienoic acid) bonded thereto and joined via CH2 linkage groups 
The inventive compositions are formed by providing a dispersion of the compounds of Formula I in a dispersant (preferably an organic solvent such as ethyl lactate), and adding an acid (such as p-toluenesulfonic acid) to the dispersion either prior to or simultaneous to heating of the dispersion to a temperature of at least about 70xc2x0 C., and preferably at least about 120xc2x0 C. The quantity of acid added should be from about 0.001-1 moles per liter of dispersant, and preferably from about 0.01-0.5 moles of acid per liter of dispersant. Furthermore, the heating step should be carried out for at least about 2 hours, and preferably from about 4-6 hours. In applications where only benzoguanamine-based moieties are utilized, the heating step should be carried out for a time period of less than about 7 hours, and preferably from about 5.5-6.5 hours.
Heating the starting compounds under acidic conditions causes the compounds to polymerize by forming the previously described linkage groups. The polymers resulting from the heating step should have an average molecular weight of at least about 1,000 Daltons, preferably at least about 5,000 Daltons, and more preferably at least about 5,000-20,000 Daltons. Furthermore, about 12 hours after the heating step the resulting anti-reflective composition should have a decrease of at least about 20%, preferably at least about 40%, and more preferably from about 40-70% in methoxymethylol (xe2x80x94CH2OCH3) groups than were present in the starting dispersions of Formula I compounds, with the quantity of methoxymethylol groups being determined by the titration procedure as herein defined.
It will be appreciated that the inventive polymer compositions provide significant advantages over prior art compositions in that the polymerized compositions alone act as conventional anti-reflective coating polymer binders, cross-linking agents, and chromophores, thus greatly simplifying the anti-reflective coating system.
In applications where enhanced light absorbance is desired, a chromophore (e.g., 2,4-hexadienoic acid, 3-hydroxy-2-naphthoic acid) can be mixed with the starting dispersion prior to acid and heat treatment. During subsequent acid treatment, the chromophore will chemically bond to the monomers during polymerization.
The resulting polymerized composition is mixed with a solvent to form an anti-reflective coating composition. Suitable solvents include propylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, ethyl lactate, and cyclohexanone. The anti-reflective coating composition is subsequently applied to the surface of a substrate (e.g., silicon wafer) by conventional methods, such as by spin-coating, to form an anti-reflective coating layer on the substrate. The substrate and layer combination a is baked at temperatures of at least about 160xc2x0 C. The baked layer will generally have a thickness of anywhere from about 500 xc3x85 to about 2000 xc3x85.
In an alternate embodiment, an anti-reflective composition is formed by preparing a dispersion including, in a dispersant (e.g., propylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, ethyl lactate), a quantity of the compound of Formula I and a polymer having cross-linking sites therein. The composition should comprise at least about 1.5% by weight of the polymer, and preferably from about 2.0-20% by weight of the polymer, based upon the total weight of the solids in the composition taken as 100% by weight. The molecular weight of the polymer is at least about 2,000 Daltons, and preferably from about 5,000-100,000 Daltons. The cross-linking sites on the polymer preferably comprise a cross-linking group selected from the group consisting of hydroxyl, carboxylic, and amide groups. The most preferred polymers include cellulose acetate hydrogen phthalate, cellulose acetate butyrate, hydroxypropyl cellulose, ethyl cellulose, polyesters, polyacrylic acid, and hydroxypropyl methacrylate.
In this embodiment, it is not necessary to heat the dispersion. However, as was the case with the first embodiment, the composition preferably includes an acid such as p-toluenesulfonic acid. Advantageously, it is not necessary to add a chromophore to the composition as the compound of Formula I also functions as a light-absorber. Thus, the composition is preferably essentially free (i.e., less than about 0.5% by weight, preferably less than about 0.1% by weight, and more preferably about 0% by weight) of any added chromophores.
In either embodiment, low molecular weight (e.g., less than about 13,000 Daltons) polymeric binders can be utilized in the dispersion (after heating and acidification steps in the case of the first embodiment) to assist in forming highly planar layers. Alternately, a high molecular weight polymeric binder (e.g., acrylics, polyester, or cellulosic polymer such as cellulose acetate hydrogen phthalate, hydroxypropyl cellulose, and ethyl cellulose) having a molecular weight of at least about 100,000 Daltons can be mixed with the starting dispersion (also after heating and acidification steps in the case of the first embodiment) to assist in forming conformal layers. This will result in an anti-reflective layer having a percent conformality of at least about 60%, even on topographic surfaces (i.e., surfaces having raised features of 1000 xc3x85 or greater and/or having contact or via holes formed therein having hole depths of from a about 1000-15,000 xc3x85).
As used herein, percent conformality is defined as:
100xc2x7|(thickness of the film at location A)xe2x88x92(thickness of the film at location B)|/(thickness of the film at location A),
wherein: xe2x80x9cAxe2x80x9d is the centerpoint of the top surface of a target feature when the target feature is a raised feature, or the centerpoint of the bottom surface of the target feature when the target feature is a contact or via hole; and xe2x80x9cBxe2x80x9d is the halfway point between the edge of the target feature and the edge of the feature nearest the target feature. xe2x80x9cFeaturexe2x80x9d and xe2x80x9ctarget featurexe2x80x9d is intended to refer to raised features as well as contact or via holes. As also used in this definition, the xe2x80x9cedgexe2x80x9d of the target feature is intended to refer to the base of the sidewall forming the target feature when the target feature is a raised feature, or the upper edge of a contact or via hole when the target feature is a recessed feature. Percent planarization is defined as:
100xe2x88x92% conformality.
Regardless of the embodiment, anti-reflective layers formed according to the invention will absorb at least about 90%, and preferably at least about 95%, of light at wavelengths of from about 190-260 nm. Furthermore, the anti-reflective layers have a k value (i.e., the imaginary component of the complex index of refraction) of at least about 0.2, and preferably at least about 0.5, at the wavelength of interest. Finally, the anti-reflective layers have high etch rates, particularly when melamine is utilized. The etch selectivity to resist will be at least about 1.5, and preferably at least about 2.0 when CF4 is used as the etchant.